1. Field of the Invention
The present invention relates to a method of selecting a frame size of image data in data transmission and a system for executing the same.
2. Description of the Related Art
Recently, with promotion of office automation and the expansion of communication networks, the facsimile apparatuses have rapidly become popular. Further, the public communication networks have been rapidly improved and expanded, so that the data transmission system has found additional various uses such as public service.
To cope with the above situations, 1988 CCITT (International Telegraph and Telephone Consultative Committee) Recommendation T.4 ANNEX A and T.30 ANNEX A prescribe an error correction system anew. Some manufacturers begin to manufacture facsimile apparatuses with the error correcting functions based on the recommendation concerning the error correction system.
The recommended error correction system uses message formats prescribed in the CCITT recommendation T.4 ANNEX A, and performs data transmission according to the transmission procedure under T.30 ANNEX A (see FIG. 1). For the message transmission, the error correction system employs high level data link control (HDLC) frame structure, and an automatic repeat request (ARQ) system in which the data frame based on the half duplex system is repetitively transmitted.
In the transmitter (calling side), the encoded image data is divided into several frames. The framed image data are transmitted, together with the frame numbers. A frame size used is 256 or 64 octet by a digital command signal (DCS) prescribed in the T.30 ANNEX A. The frame size normally used is 256 octet.
When a data error occurs in the frame, a receiver (called side) transmits a partial page request (PPR) under the T.30 ANNEX A to the calling side, and requests the calling side to retransmit the frame containing the error.
Upon reception of the PPR, the calling side retransmits again the requested frame to the called side.
Through such a transmission procedure, the error in the frame is corrected, and the image data not including error is transmitted.
In the facsimile apparatus having the above error correcting function, the frame size of the image data is set at 256 or 64 octet by the DCS when it is transmitted, or may be manually set by an operator every time the data transmission is performed.
A rate of transmission error occurrence in the facsimile communication depends on various conditions such as transmission line conditions and transmission distance. Accordingly, the number of transmission errors is also dependent on those conditions. For example, in the facsimile communication having a high rate of transmission error occurrence, when 256 octet is used for the frame size, an amount of retransmitted data is increased due to the number of the error occurrence, and hence the transmission time is also extended.
To avoid this, for example, the frame size is set at 64 octet, which is smaller than the normal frame size. If the frame size is used, in the above case of the facsimile communication having a high rate of transmission error occurrence, the amount of the retransmitted data is decreased because of the small frame size, 64 octet and, therefore, the retransmission time of data is not extended.
In the facsimile apparatus having the above function, the frame size is manually set by an operator. In the facsimile communication using the facsimile apparatus, therefore, an operator checks a transmission distance from a calling side to a called side, transmission time, and the like, and determines a suitable frame size on the basis of the check results. Thus, since the frame size is selected manually, a frame size may be mistakenly selected.
In this respect, there is a demand of new facsimile apparatus capable of automatically selecting a suitable frame size of the transmitted image data on the basis of the error causing factors in the used transmission line.